Colt has not offered a AR15 in 308
$800-$2,000.
$1599
250-2500 USD depending on the EXACT model.
As of today (10/18/08) $900 ~ $1,370 depending on condition.
Try AR15.com Try AR15.com
To a collector of Colt oddities, it might be worth a small premium of 50-100 USD.
If it is one of the recent Colt .22LR AR15 rifles, then the answer is no - they are completely different internally.
Yes
1959
The US Army presently uses the M4 carbine, a shorter and much modified version of the Armalite AR15.
By it's serial number. Go to www.proofhouse.com and under 'long rifles' listing look for AR15 Sporter